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   On-board energy storage in DC systems  evaluated  
Modern energy storage devices permit the storage of braking energy on-board for use in subsequent acceleration phases. Especially in DC system, where energy losses in the distribution network are high, this could be an interesting alternative to feeding back energy into the supply system. Furthermore reduced peak load is beneficial for system capacity, voltage stability and energy costs.
Technology field: Regenerative braking and energy management
close main section General information
  close sub-section Description


Modern energy storage devices permit the storage of braking energy on-board for use in subsequent acceleration phase. Especially in DC systems, where energy losses in the distribution network are high, this could be an interesting alternative to feeding back energy into the supply system.

Without on-board energy storage much of the potential for brake energy recovery is not exploited because the system is often not able to take up the power. By means of energy storage, brake energy recovery becomes independent of load situation in the supply system. This way the recovery rates and energy savings may be raised substantially.

Furthermore the peak loads of the supply system are reduced, with results beneficial both for system capacity, voltage stability and energy costs.

Energy storage in the driving cycle

Figure 1 shows the use of energy storage in a typical driving cycle consisting of the phases acceleration – constant speed – braking – standstill.


Figure 1: Driving Cycle and Storage Cycle. Line 1: Speed v, tractive effort T. Line 2: Power P, Energy E. Line 3: Voltage U, current i of a supercapacitor for energy storage

Driving cycle.jpg

Source: Hentschel et al. 2000

During braking phases the kinetic energy of the vehicle is transformed into another energy type (e.g. electrostatic energy in the case of a capacitor) and stored in the storage device. When the vehicle stands still the energy storage device should be fully charged to be able to deliver energy during the subsequent acceleration phase. The energy supply during acceleration is supported by the stored energy. The energy management system should be designed in such a way that the external energy supply never needs to deliver the full accelerating power (an important condition to downsize energy supply! cf. below). When driving at maximum speed the storage device should be completely discharged.

Integration into drive system

Figure 2: Integration into drive system


Source: IZT

Choice of energy storage device

The choice of the best energy storage device heavily depends on the individual vehicle and service type. The following table shows the main characteristics to be looked at in an individual application context and the corresponding storage parameters.

Characteristics of application context Corresponding parameter of storage device
Braking time Charging time / power density
Braking energy Energy density
Drive cycles in lifetime Product life / reliability
 Source: IZT

Table 1 gives the relevant characteristics for some trains.

Table 1: Relevant characteristics of some trains Regional Multiple Unit (EMU) Suburban EMU

Regional Multiple Unit (EMU)

Suburban EMU      (DC, 4 cars)

Light rail vehicle

Mass of train (brutto)

250 t

160 t

39 t

Top speed

160 km/h

100 km/h

70 km/h

Drive power

4000 kW

1200 kW

300 kW

Maximum traction effort

250 kN

180 kN

60 kN

Braking power

4800 kW

3000 kW

900 kW

Maximum braking effort

270 kN

200 kN

100 kN

Stored kinetic energy

70 kWh

16 kWh

2 kWh

Braking time

50 s

15 s

9 s

Drive cycles per year




Drive cycles in lifetime (106)




Source: Hentschel et al. 2000

The Ragone diagram plotting energy density against power density is a convenient means to compare different storage technologies and assess their suitability for different vehicles. Figure 3 shows the position of relevant storage technologies in the Ragone diagram and the charging times. They are labelled by train types with corresponding braking times.

Figure 3: Ragone diagram and charging times (corresponding to braking times of different trains)


Source: IZT, data mainly from: Hentschel, Müller et al. 2000.

Batteries have very high energy densities but low power densities leading to very high charging times. Nickel metal hydride batteries have sufficient power densities but do not meet the lifespan requirements. Batteries are therefore presently not the first choice for on-board storage.

Flywheels have both high power and high energy densities and are therefore ideally fitted to most situations, especially regional trains.

Supercaps which have made considerable development progress in recent years are beginning to become attractive for railway applications especially light rail urban vehicles, but there are also high-energy supercaps that could be interesting for storage solutions on heavy vehicles.

Superconducting magnetic energy storage (SMES) is of the extreme low-energy high-power type and is therefore not an attractive option for brake energy storage.

close main section General criteria
  close sub-section Status of development: research & experiments
    (no details available)
  Time horizon for broad application: 5 - 10 years
    (no details available)
  Expected technological development: highly dynamic
    Technological developments will come mainly from storage technologies (cf. fly-wheel, double-layer capacitors, batteries and SMES).
    Energy savings
  Benefits (other than environmental): medium

Peak load levelling

The load peaks of the supply system are equalized and thus reduced, which improves voltage stability.

Energy price

Furthermore reduced peak demands save energy costs since peak load energy is especially expensive.

  Barriers: (no data)


Storage technologies are starting to become a mature technology but lifespan is often still a problem.

Vehicle mass

Storage system increases vehicle mass.


High investment costs and long payback periods.


Operators are reluctant to buy trains with higher complexity, since this could decrease availability and reliability and always requires additional know-how.

    Success factors:
    (no details available)
  Applicability for railway segments: medium
    Type of traction:  electric - DC
    Type of transportation:  passenger - regional lines, passenger - suburban lines
    It is evident, that energy storage shows best benefits and payback for local and regional trains with frequent stops (many storage cycles!). In AC systems on-board energy storage is possible as well but usually not attractive since feedback into supply system is much more effective than in DC systems. An exception could be some regional AC networks with frequent stops but low train density.
    Grade of diffusion into railway markets:
  Diffusion into relevant segment of fleet: 0 %
  Share of newly purchased stock: 0 %
    (no details available)
  Market potential (railways): low
    (no details available)


close main section Environmental criteria
  close sub-section Impacts on energy efficiency:
  Energy efficiency potential for single vehicle: > 10%
  Energy efficiency potential throughout fleet: (no data)

Figure 4 shows the energy saving effect of energy storage on different train types. However, these figures have to be treated with some care. They are a result from theoretical calculations by manufacturers and are quite optimistic. Even so, the figures indicate the huge saving potential offered by energy storage in DC systems.

As can be seen in Figure 4, the energy saving potential comes from two effects:

  • recovery and reuse of brake energy ("vehicle only" in the figure)
  • reduction of losses in the distribution network due to reduced peak currents (Losses are proportional to the square of the current!).

Figure 4: Energy savings by energy storage for different trains (% of total energy consumption)

Energy storage

Source: IZT, data from: Hentschel, Müller 2000

  Other environmental impacts: neutral


close main section Economic criteria
  close sub-section Vehicle - fix costs: high
    Investment costs depend on the storage technology and the system layout chosen but are generally high.
  Vehicle - running costs: significant reduction
    The (relative) savings in energy costs are higher than the (relative) energy savings themselves since the equalizing of the load peaks substantially cuts the energy price (which is determined by peak demands).
  Infrastructure - fix costs: none
    (no details available)
  Infrastructure - running costs: unchanged
    (no details available)
  Scale effects: high
    Scale effects for storage technologies.
  Amortisation: 2 - 5 years

Amortisation of energy storage systems is in the range of 10 to 30% of the vehicle lifetime, that is 3 to 10 years, depending on the vehcile and the storage type. It could be appreciably reduced by scale effects.

In many cases, amortisation is more favourable for stationary storage.

no data available Application outside railway sector (this technology is railway specific)
close main section Overall rating
  close sub-section Overall potential: promising
  Time horizon: mid-term
    Storage systems for DC systems are an interesting option for energy efficiency. Payback times for present storage technologies are critical, but are expected to improve in the future.
References / Links:  Hentschel et al. 2000
Related projects:  Studies performed on energy storage systems;  THALES: hybrid tram train with on-board ultracapacitors
Contact persons:
 date created: 2002-10-09
© UIC - International Union of Railways 2003
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